This invention relates to selective conversion of alkenes and more particularly relates to selective isomerization of normal alkenes, such as normal butenes, using a catalyst composition containing high concentrations of AMS-1B crystalline borosilicate molecular sieve.
In many instances it is desirable to convert an alkene such as normal butene by mechanisms such as structural isomerization and double bond shift. Such converted alkenes then can be reacted further, such as by polymerization or oxidation, to form useful products. Normal alkenes containing four carbon atoms include 1-butene, trans-2-butene and cis-2-butene and are relatively inexpensive feedstocks. Isobutylene is a branched four-carbon alkene useful in the manufacture of polyisobutylenes which can have various properties depending on the manner of polymerization. For example, both crystalline polyisobutylene and viscous polyisobutylene can be manufactured according to well-known processes in the art. In addition, isobutylene is used in the manufacture of methyl-t-butyl ether which is useful as an octane booster in gasoline. Conventionally, butylenes, including isobutylene, are obtained as a by-product from refinery processes such as catalytic or thermal cracking units. For manufacture and uses of butylenes, see Kirk-Othmer, "Encyclopedia of Chemical Technology," Third Edition, Vol. 4, pp. 346-375, incorporated herein by reference.
Zeolitic materials, both natural and synthetic, are known to have catalytic capabilities for many hydrocarbon processes. Zeolitic materials typically are ordered porous crystalline aluminosilicates having a definite structure with cavities interconnected by channels. The cavities and channels throughout the crystalline material generally are uniform in size allowing selective separation of hydrocarbons. Consequently, these materials in many instances are known in the art as "molecular sieves" and are used, in addition to selective adsorptive processes, for certain catalytic properties. The catalytic properties of these materials are affected to some extent by the size of the molecules which selectively penetrate the crystal structure, presumably to contact active catalytic sites within the ordered structure of these materials.
Generally, the term "molecular sieve" includes a wide variety of both natural and synthetic positiveion-containing crystalline zeolite materials. They generally are characterized as crystalline aluminosilicates which comprise networks of SiO.sub.4 and AlO.sub.4 tetrahedra in which silicon and aluminum atoms are cross-linked by sharing of oxygen atoms. The negative framework charge resulting from substitution of an aluminum atom for a silicon atom is balanced by positive ions, for example, alkali-metal or alkaline-earth-metal cations, ammonium ions, or hydrogen ions.
Prior art developments have resulted in formation of many synthetic zeolitic crystalline materials. Crystalline aluminosilicates are the most prevalent and, as described in the patent literature and in the published journals, are designated by letters or other convenient symbols. Examples of these materials are Zeolite A (U.S. Pat. No. 2,882,243), Zeolite X (U.S. Pat. No. 2,882,244), Zeolite Y (U.S. Pat. No. 3,130,007), Zeolite ZSM-4 (U.S. Pat. No. 3,578,723), Zeolite ZSM-5 (U.S. Pat. No. 3,702,886), Zeolite ZSM-11 (U.S. Pat. No. 3,709,979), Zeolite ZSM-12 (U.S. Pat. No. 3,832,449), Zeolite NU-1 (U.S. Pat. No. 4,060,590) and others.
Boron is not considered a replacement for aluminum or silicon in a zeolitic composition. However, recently a new crystalline borosilicate molecular sieve AMS-1B with distinctive properties was disclosed in U.S. Pat. Nos. 4,268,420 and 4,269,813, incorporated by reference herein. According to these patents AMS-1B can be synthesized by crystallizing a source of an oxide of silicon, an oxide of boron, an oxide of sodium and an organic template compound such as a tetra-n-propylammonium salt. The process of this invention uses AMS-1B crystalline borosilicate molecular sieve.
Hydrocarbon conversion processes are known using other zeolitic materials. Examples of such processes are dewaxing of oil stock (U.S. Pat. Nos. 3,852,189, 4,211,635 and Reissue 28,398); conversion of lower olefins (U.S. Pat. Nos. 3,965,205 and 3,960,978 and European Patent Application No. 31,675); aromatization of olefins and aliphatics (U.S. Pat. Nos. 3,761,389, 3,813,330, 3,827,867, 3,827,868, 3,843,740, 3,843,741 and 3,914,171); hydrocracking and oligomerization of hydrocarbons (U.S. Pat. Nos. 3,753,891, 3,767,568, 3,770,614 and 4,032,432); conversion of ethane to aromatics and C.sub.3.sup.+ hydrocarbons (U.S. Pat. No. 4,100,218); conversion of straight-chain and slightly branched chain hydrocarbons to olefins (U.S. Pat. Nos. 4,309,275 and 4,309,276); and conversion of C.sub.4 paraffins to aromatics (U.S. Pat. No. 4,291,182).
Conversion of C.sub.4 hydrocarbons under some conditions is described in commonly assigned U.S. patent application Ser. No. 422,821 in the name of Nevitt, Sikkenga and Jerome, Ser. No. 422,743 in the name of Peters and Klotz, Ser. No. 422,822 in the name of Melquist, and Ser. No. 422,744 in the name of Nevitt and Jerome, all filed of even date herewith and all incorporated by reference herein. The improvement described herein is the discovery that catalyst formulations in which hydrogen form AMS-1B crystalline borosilicate molecular sieve comprises a major portion of the catalyst formulation, and a binder such as alumina comprises a minor portion, show high alkene isomerization activity such that low hydrocarbon partial pressures, high temperatures and short contact times can be used to produce high selectivity to isoalkenes such as isobutylene.
A method to manufacture isobutylene from a normal alkene such as n-butene would be desirable and a method that would isomerize a carbon structure in one step without excessive losses to undesirable by-products would be especially desirable. Further, a process that selectively converts normal butenes to more useful and valuable products such as isobutylene would be advantageous.